Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional beamline ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. Energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Beamline ion implanters are typically designed for efficient operation at relatively high implant energies and may not function efficiently at the low energies required for shallow junction implantation.
Plasma doping systems have been studied for forming shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen, which functions as a cathode and is located in a process chamber. An ionizable process gas containing the desired dopant material is introduced into the chamber, and a voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied pulse causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low implant energies can be achieved. Plasma doping systems are described, for example, in U.S. Pat. No. 5,354,381, issued Oct. 11, 1994 to Sheng; U.S. Pat. No. 6,020,592, issued Feb. 1, 2000 to Liebert et al.; and U.S. Pat. No. 6,182,604, issued Feb. 6, 2001 to Goeckner et al.
In the plasma doping systems described above, the applied voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma systems, known as plasma immersion systems, continuous or pulsed RF energy is applied to the process chamber, thus producing a continuous plasma. At intervals, negative voltage pulses, which may be synchronized with the RF pulses, are applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer.
The distribution and frequency of different ion mass-to-charge ratios in the plasma has a fundamental impact on the implant dose and the implant depth profile distribution in plasma ion implantation. Many factors can impact the ion mass distribution in plasma ion implantation systems, including process chamber wall conditions, electron emission coefficients of targets and process chamber components, oxide and photoresist coatings on wafers, etc. To obtain a repeatable plasma ion implantation process, variation in these factors should be detected and compensated or neutralized, so that a repeatable ion mass distribution and intensity can be obtained for a process. This permits a repeatable ion dose and dopant depth distribution to be obtained in a plasma ion implantation process.
Mass analysis has been employed in traditional beamline ion implantation systems. However, mass analysis has been abandoned in plasma ion implantation systems in order to obtain the benefits of very high throughput in plasma-based processing. U.S. Pat. No. 6,101,971, issued Aug. 15, 2000 to Denholm et al., discloses the use of optical emission spectroscopy and mass analysis in a plasma ion implantation system. This patent does not teach the use of mass analysis for in-situ plasma state measurement or process control in plasma ion implantation systems.
In plasma ion implantation systems, short DC voltage pulses (approximately 1 to 100 microseconds) are applied to a substrate immersed in a plasma. These pulses accelerate positive ions in the plasma toward the target, causing ion implantation. The voltage and current waveforms of the DC implant pulses and variations in these waveforms can indicate problems in the implant process. Typical monitoring of plasma ion implantation includes a residual gas analyzer or optical emission spectroscopy instrumentation. This type of plasma monitoring is performed on a time scale that is too long to detect transient changes in the critical voltage and current waveforms of the DC pulses used in plasma ion implantation. The valuable process monitoring information contained in these waveforms is lost when typical plasma monitoring techniques are utilized.